FORT WORTH, Texas Republican candidate Donald Trump on Friday won the surprise endorsement of New Jersey Governor Chris Christie, the most prominent mainstream Republican to get behind the former reality TV star's White House campaign.

Christie said the billionaire front-runner has the best chance of beating Democrat Hillary Clinton in the Nov. 8 U.S. presidential election - although Clinton has yet to secure her party's nomination.

The endorsement gives Trump a further lift before next week's Super Tuesday nominating contests. It comes just a day after he took a battering from his two main rivals at a televised Republican debate.

Trump's unorthodox candidacy has stirred controversy and shaken the Republican Party at its roots, but an increasing number of senior Republicans are becoming resigned to the idea he will be their candidate in November.

Trump is "rewriting the playbook," said Christie, 53, who until two weeks ago was himself a rival for the Republican nomination. Christie dropped out after failing to muster much support for his candidacy.

Trump, 69, who has never held public office, has campaigned as a political outsider. He is riding a wave of voter anger at the slow economic recovery, illegal immigration and what he says is America's diminishing role in the world.

"The best person to beat Hillary Clinton in November on that stage last night is undoubtedly Donald Trump," Christie told a news conference on Friday, a day after the last Republican candidates' debate before Super Tuesday.

The debate marked a new, more aggressive approach for U.S. Senator Marco Rubio, 44, who has emerged as the Republican establishment's challenger to Trump. The other main challenger at the debate was U.S. Senator Ted Cruz of Texas.

Trump has unsettled mainstream Republicans by winning three straight nominating contests - in New Hampshire, South Carolina and Nevada. Polls show he is likely to win big in key primaries on Tuesday.

"Since I started this whole thing I've been practically Number 1," Trump said on Friday at a rally in Texas.

The 11 Republican nominating contests on Tuesday have a total of almost 600 delegates at stake, and could set Trump up to clinch the presidential nomination.

Reuters/Ipsos polling data on Friday showed Trump ahead nationally in the Republican race with support at 44.2 percent, followed by Cruz at 20.7 percent and Rubio in third place at 14 percent.

On the Democratic side, former Secretary of State Clinton is battling U.S. Senator Bernie Sanders of Vermont. Clinton and Sanders have been in a dead head over the past week, the Reuters/Ipsos data shows.

RUBIO ATTACKS

Trump has vowed to build a wall on the U.S.-Mexican border to halt illegal immigration, called for a temporary ban on Muslims entering the United States and promised to take a tough stance on trade against China.

He was combative at a rally on Friday. He mocked Rubio, referred to violent Islamist militants as "these animals" and promised to defend Americans' constitutional right to bear arms.

"We're going to build up our military, we're going to knock out ISIS. We're going to knock out ISIS fast," he said, referring to the Islamic State militant group.

Wielding a water bottle as a prop, Trump made fun of Rubio for an awkward incident in which the senator grabbed for a drink of water off camera during an important televised speech in 2013.

Rubio and Cruz ganged up on Trump at Thursday's debate in Houston in a last-ditch bid to keep him from winning in states on Super Tuesday.

Rubio on Friday again took aim at Trump.

"He’s a con man who’s taking advantage of people's fears and anxieties about the future, portraying himself as some sort of strong guy," Rubio told reporters in Oklahoma. "He’s not a strong guy. He’s never faced real adversity before."

PredictWise, a research project that analyzes opinion polls and betting markets, said Trump would comfortably win among Republicans in all but one of the 11 Super Tuesday states that it measured. Cruz, 45, is likely to win in his home state of Texas, PredictWise said.

Rubio's home state of Florida is not part of the Super Tuesday contests.

PredictIt, based out of Victoria University in Wellington, New Zealand, on Friday gave Trump a 73 percent chance of winning the nomination compared with a peak 75 percent chance two days earlier.

Trump's swipes at rival candidates and heated exchanges with journalists and others have for months bolstered his standing in nominating contests and opinion polls.

In a post on Twitter, Trump took aim at Rubio, a first-term senator, for his debate performance.

"Lightweight Marco Rubio was working hard last night. The problem is, he is a choker, and once a choker, always a chocker (sic)! Mr. Meltdown."

Republican strategist Doug Heye said Christie may have opened the door for more mainstream Republican endorsements of a man whose chances of winning the White House were seen as next to nil a year ago.

“If you’re the Trump campaign this is obviously very good news and it gives permission for others to endorse. But it also makes it hard (for Trump) to make the outsider argument," he said.

Glenn Hubbard, who had been an adviser to the campaign of former Florida Governor Jeb Bush and was chair of the Council of Economic Advisers during the George H.W. Bush administration, said he planned to keep up steady criticism of Trump on economic issues.

"I think it is time for serious people to stand up and be counted. The next few weeks come very quickly," said Hubbard, who published a column in the Boston Globe on Friday criticizing Trump.

Hubbard, now dean of the business school at Columbia University, told Reuters he worried Trump's comments already hurt the country's image abroad and would hobble his ability to govern if elected.

Next In U.S. Elections

WASHINGTON Senate Republican leader Mitch McConnell told President Barack Obama's Supreme Court nominee Merrick Garland by phone on Wednesday that Republicans will not act on his nomination or meet with him, a McConnell spokesman said."Rather than put Judge Garland through more unnecessary political routines orchestrated by the White House, the leader decided it would be more considerate of the nominee’s time to speak with him today by phone," McConnell spokesman Don Stewart said in

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